Influenza viruses (types A, B, and C) are members of the orthomyxoviridae family that cause influenza. Type A influenza viruses infect birds and mammals, including humans, whereas types B and C infect humans only. Influenza viruses are roughly spherical enveloped viruses of about 8-200 nm diameter that contain segmented negative sense genomic RNA. The envelope contains rigid structures that include hemagglutinin (HA) and neuraminidase (NA). Combinations of HA and NA subtypes, which result from genetic resortment, are used to characterize viral isolates. Generally, influenza viral isolates are identified by nomenclature that includes type, location, isolate number, isolation year, and HA and NA subtypes (e.g., “A/Sydney/7/97(H3N2)” refers to type A, from Sydney, isolate 7, in 1997, with HA 3 and NA 2 subtypes). Minor genetic changes that produce antigenic drift may cause influenza epidemics, whereas genetic changes that result in a new HA or NA subtype produce antigenic shift that may cause a pandemic. Analysis of human influenza virus A infections has shown that a few HA and NA combinations were clinically significant in causing pandemics during the 1900s, i.e., H1N1 in 1918, H2N2 in 1957, and H3N2 in 1968.
Influenza viruses that infect birds (e.g., chickens, ducks, pigeons) use combinations of H5, H7 or H9 with any of N1 to N9. Since 1997, avian influenza viruses that have infected humans have included H5N1, H9N2, H7N2, and H7N7 viruses. Even limited human infections caused by an avian influenza virus raise concern for a potential pandemic, resulting in quarantines, and intentional destruction of large numbers of fowl, with accompanying hardship. An avian influenza virus, or variant derived therefrom, that efficiently transfers by human-to-human contact could cause a pandemic (Li et al., 2003, J. Virol. 77(12): 6988-6994).
Human influenza viruses produce highly contagious, acute respiratory disease that results in significant morbidity and economic costs, with significant mortality among very young, elderly, and immuno-compromised subpopulations. Avian influenza infections in humans generally have a high mortality rate. A typical influenza virus infection in humans has a short incubation period (1 to 2 days) and symptoms that last about a week (e.g., abrupt onset of fever, sore throat, cough, headache, myalgia, malaise and anorexia), which may lead to pneumonia. Optimal protection against infection requires annual inoculation with a vaccine that includes a compbination of types A and B of the most likely subtypes for that year, based on global epidemiological surveillance. To be effective in treatment, pharmaceuticals that block viral entry into cells or decrease viral release from infected cells must be administered within 48 hrs of symptoms onset.
A variety of methods have been used to detect influenza viruses clinically. Viral culture in vitro (in monkey kidney cells) followed by visual analysis and/or hemadsorption using microbiological methods can detect influenza viruses A and B in specimens (e.g., nasopharyngeal or throat swab, nasal or bronchial wash, nasal aspirate, or sputum). Other detection tests include immunofluorescence assays (IFA), enzyme immunoassays (EIA), and enzyme-linked immunosorbent assays (ELISA) that use antibodies specific to influenza virus antigens. Examples include a sandwich microsphere-based IFA that uses influenza A- or B-specific monoclonal antibodies and flow cytometry (Yan et al., 2004, J. Immunol. Methods 284(1-2): 27-38), monoclonal antibody-based EIA tests (DIRECTIGEN® FLU A and DIRECTIGEN® FLU A+B, Becton, Dickinson and Co., Franklin Lakes, N.J., and QUICKVUE® Influenza Test, Quidel, San Diego, Calif.), and an immunoassay that produces a color change due to increased thickness of molecular thin films when an immobilized antibody binds an influenza A or B nucleoprotein (FLU OIA®, Biostar Inc., Boulder, Colo.). Another chromagenic assay detects viral NA activity by substrate cleavage (ZSTAT FLU®, ZymeTx, Inc., Oklahoma City, Okla.). Assays are known that rely on reverse-transcriptase polymerase chain reactions (RT-PCR) to amplify influenza viral sequences to detect influenza A and B viruses (e.g., Templeton et al., 2004, J. Clin. Microbiol. 42(4):1564-69; Frisbie et al., 2004, J. Clin. Microbiol. 42(3):1181-84; Boivin et al., 2004, J. Clin. Microbiol., 42(1):45-51; Habib-Bein et al., 2003, J. Clin. Microbiol. 41(8):3597-3601; Li et al., 2001, J. Clin. Microbiol. 39(2):696-704; van Elden et al., 2001, J. Clin. Microbiol. 39(1): 196-200; Fouchier et al., 2000, J. Clin. Microbiol. 38(11):4096-101; Ellis et al., 1997, J. Clin. Microbiol. 35(8): 2076-2082; PCT Nos. WO 2004 057021, WO 02 00884, WO 00 17391, and WO 97/16570, EP Publ. No. 1 327 691 A2, U.S. Pat. No. 6,015,664, and PROFLU-1™ and HEXAPLEX™ tests, Prodesse, Milwaukee, Wis.). Serology detects seroconversion associated with influenza virus A or B infections by detecting antibodies present in acute and convalescent sera from patients with influenza symptoms. Detection methods have associated advantages and disadvantages related to sensitivity, specificity, assay and handling time, required equipment, and exposure of technical personnel to infectious agents with related safety requirements for laboratories and personnel. Generally, culture and serological tests require longer completion times (5 days to 2 weeks) with potentially greater exposure of technical personnel to infectious agents. Immunoassays are generally faster (30 min to 4 hrs) but often require substantial sample handling and rely on subjective determination of results by technical personnel. PCR-based amplification assays may take up to 2 days to complete and require specialized thermocycling equipment. Hence a need remains for a test that provides sensitive, specific detection of influenza virus type A and type B in a relatively short time, with a minimum of exposure of technical personnel to infectious agents, so that diagnosis is completed in sufficient time to permit effective therapeutic treatment of an infected person.